Power control (PC) is an essential function of cellular telephone systems such as CDMA systems and WCDMA systems, as well as follow on systems thereto. It is important that the power transmitted from a base station (BS) to a mobile unit (MU) be closely controlled such that it is sufficiently high enough to ensure that the required communications and performance is achieved. This is also the case with respect to power transmitted from the MU to the BS. If more power is transmitted than is required, the MU, for example, will be required to utilize more of its battery power. The BS, although not being powered by battery, does have overall power requirements that need to be met as well. Thus, by reducing the amount of total power that is required to be transmitted to the maximum number of mobile units that could possibly be interfaced with the BS, a more efficient system could be utilized with optimized power supplies, etc.
Power control is facilitated utilizing only the traffic and access channels. The power levels transmitted from MUs to their BSs are very closely controlled, typically utilizing multiple control loops to ensure that just enough, but not too much, power is transmitted. One loop is utilized for open loop control and it is based on the level of power received over the total physical channel bandwidth. A second loop is comprised of a closed loop which utilizes measurements of power on reverse traffic channels to determine if the reverse-link is approximately at the level required. If it is not, a one-bit control message is sent out on the forward traffic channel to adjust the power of a particular link. A third loop can be utilized, usually called the outer loop, which appraises the overall performance of the closed loop using the reverse-link frame quality statistics. Internally, the parameters that are examined are typically such things as the Signal-to-Interference Ratio (SIR) and the bit error rate (BER).
One problem that exists with respect to a cellular telephone system which has a plurality of MUs disposed in the proximity to a particular BS is that the MUs can migrate into different microenvironments. For example, two MUs can be separated by a distance of 10 feet and be in a completely different environment due to the surrounding features of that environment. For example, one person may be outside of a building and the other person may be 10 feet away on the inside of the building looking out of a window. The communication properties between those two MUs are significantly different. This can be further exacerbated in a CDMA system wherein both MUs receive on substantially the same frequency utilizing only Welch codes to distinguish two people talking at the same time. This is facilitated by controlling the power on a per user basis. When an individual steps inside of a building the attenuation caused by the building will be compensated for by the MU requesting higher power to be transmitted from the BS and for the BS requesting higher power to be transmitted from the MU. This is fairly conventional.
One other factor with respect to these microenvironments is that the characteristics of the electromagnetic wave are varied as a result of the surrounding environment. Some of these characteristics are due to reflections which can change the polarization. For example, if a signal is reflected from a building, polarization could be rotated from a conventional vertical polarization to lead or lag that polarization. Since the handset corresponding to the MU is typically on the average expecting vertical polarization, this will result in some attenuation which will require a power increase in the overall band of interest in order to gain acceptable communications performance. This is also the case when entering the building, as the building itself will constitute a phase shifter. This is in addition to the attenuation of the building itself. The only solution at the present time is to utilize the power control features of the cellular communication system to facilitate the change.